Britain must act to prevent it slipping into the scientific equivalent of the dark ages, Sir Peter Williams, president of the British Association, said yesterday.

Launching the association's festival of science, which begins at the University of Salford today, Sir Peter said he foresaw problems in sustaining scientific advancement, as fewer pupils chose biology, chemistry and physics courses at school or took science degree courses. The schism between sciences and humanities - the "two cultures" identified by the scientist and novelist CP Snow more than 40 years ago - was still with us.

He said: "Today, as perhaps never before, the public's support - even consent - is essential if new avenues for scientific research are to be sustainable long term. In its turn, the public - and especially government - have a responsibility to ensure that we do not slip back into some scientific equivalent of the dark ages. We have to stress the overwhelming benefits science brings, rather than always tending to look to the dark side."

For almost 30 years computers had halved in size and doubled in power every one to two years. But such gains could not be sustained, and the semiconductor world was about to become more complex. The pre-war discovery of penicillin, and the DNA breakthroughs, had raised public faith that bioscience could always devise new treatments; that there would always be an antidote for everything.

"But did anyone ask the bugs? Faster than a combinatorial chemist can devise new molecules, nature itself, it would seem, can devise ever more successful opponents, from HIV to Sars. Who knows what is coming next? Is our dependency on ever more sophisticated and vastly more expensive biopharmaceutical products a sustainable basis for future healthcare needs?

"Or is science in many sectors, including the two I have just cited, in danger of becoming a little reminiscent of oil prospecting, founded on an arrogant belief that just as soon as the applications of current knowledge appear to be approaching exhaustion, new discoveries of even greater impact will always be uncovered. Is the process sustainable?"

The other challenge to scientific advance lay in the schools. Education chiefs in Britain and many developed countries were worried because young people seemed to be turning away from science.

"It is to me nothing less than a CP Snow two-cultures-style tragedy that, at 16, those who do choose to go on to precursor of tertiary education in the UK are 'streamed' so rigidly - and it is pretty rigid whatever the schools will tell you - between the sciences and the humanities. What can we do about that? Why is it that sixth formers in the humanities do not pursue a course of study - if they are interested in history - in the history of science?"

Sir Peter continued: "You don't have to understand quantum mechanics to sit in a room with your fellow sixth formers to debate the ethics of human genomics, of stem-cell research and issues like that. If we accept this streaming ... why cannot we do something to bring these communities together?"

Modern science underpinned economic advance. Last year almost 200 new companies were formed as spin-outs from British universities, creating new jobs and linking research with industry.

"As scientists in a free society, we cannot turn our backs on this recent economic revolution. Science is, after all, increasingly expensive." Comprehensive spending reviews between 1997 and 2006 will show the science share doubling to almost £3bn per year.

"Science can still pursue that elusive dream in cosmology or biochemistry, but the linkage between the outputs from science, the needs of industry and the economic wellbeing of the nation needs to be continually demonstrated."